1. Field of the Invention
This application relates to communication cables. More particularly, this application relates to loose-tube type fiber optic cables.
2. Description of the Related Art
In the area of fiber optic cables, there are many different design features, each of which has some purpose, including fiber count, mechanical properties, environmental resistance properties, fire resistance/smoke, etc. . . . . Among the various fiber optic cable designs, mid count-designs (e.g. more than 12—less than 200 fibers) typically contain the fibers in a loose tube style arrangement. “Loose tube” is a commonly understood term designating a fiber cable design that has a jacket, at least one buffer tube inside the jacket, with at least one (usually more) UV coated optical fiber(s) loosely contained inside each buffer tube.
More particularly, the “loose” term in “loose tube” refers to the fibers being loose within buffer tube thus allowing the fibers to reside within a relatively free space. Within this free space the fibers have the ability to bend/move (such as into a sinusoidal shape or minimally helical shape) along the length of the cable, accumulating as the cable (jacket and tubes) contracts over cold temperature extremes. By allowing for this “loose” room with the buffer tubes, the fibers are able to avoid the stresses imparted by cold temperatures on the tubes and jacket, and, as such, also avoids undue attenuation.
FIGS. 1 and 2 show typical prior art mid-sized loose tube fiber optic cables for forty eight (48) fibers (FIG. 1) and seventy two (72) fibers (FIG. 2). In these figures, a jacket is provided with either four or six buffer tubes each having twelve (12) loose fibers therein. Each of the tubes are arranged around a central strength member. This arrangement with the centrally located strength member, typically a GRP (Glass Reinforced Polymer), provides various mechanical advantages to the cable Including longitudinal strength, and cold temperature contraction resistance, cable resistance, etc. . . . In the examples shown in FIGS. 1 and 2, a few (e.g. three) aramid strength strands are added to the interior of the tubes) to aid with the strength of connectors added to the fibers at the ends of tubes (i.e. once they are removed from the cable for connectorization).
FIGS. 3-5 show other types of prior art loose tube fiber optic cables, having ninety six (96) fibers (FIG. 3); one hundred and thirty two (132) fibers (FIG. 4); and one hundred and forty four (144) fibers (FIG. 5), respectively. In these figures, a jacket is provided with eight, eleven or twelve tubes, each tube having twelve (12) loose fibers therein. As with FIGS. 1 and 2, each of the tubes are arranged around a central strength member, but in this case the strength member is an up-jacketed (i.e. polymer coated) GRP (Glass Reinforced Polymer) to provide various mechanical advantages to the cable including longitudinal strength, and cold temperature contraction resistance, cable resistance, etc. . . . . An up-jacketed CSM (Central Strength Member) is usually constructed as GRP (Glass Reinforced Polymer) that may be encased within an extruded polymer jacket. As with the prior art arrangements in FIGS. 1 and 2, these examples in FIGS. 3-5, a few (e.g. three) aramid strength strands may also be added to the interior of the tube(s) to aid with the process of adding connectors to the fibers at the ends of tubes, when they are disassembled from the cable for connectorization.
Although these designs are adequate for many purposes, they have certain strength and mechanical property drawbacks, particularly with cold temperature resistance.
For example, as noted above, a normal design characteristic for a fiber optic cable is a cold temperature resistance rating. Cold temperatures cause the polymers in the jacket and buffer tubes to constrict, changing the spatial dimensions and relationships between the fibers and the polymer surrounding them. As explained in more detail below, under very cold temperatures, this constriction of polymer components eventually presses against the fibers, causing contact and excessive bending of the fibers leading to signal attenuation. A cold temperature resistance rating is the ability of a cable to withstand a certain low temperature without exceeding a certain threshold of attenuation. For example, in many designs for mid-count fiber optic cables, there is a maximum allowable attenuation of 0.3 db change under low temperature conditions (typically either 0° C., −20° C. or −40° C.).
The prior art designs of FIGS. 3-5 which have an upjacketed CSM (polymer covered central strength member) intrinsically have certain design limitations with respect to the cold temperature rating. For example, the upjacketed CSM, where upjacket encasement is extruded polymer on the CSM, has an increased cold temperature shrinkage due to the large size and the added plastic area. In other words, cold temperature shrinkage parameters for a cable is based in part on the type and total cross sectional area of the polymer in a cable, and an upjacketed CSM has additional cross sectional area of polymer, and thus increased shrinkage issues.
Another related problem to the general problem of shrinkage of the polymer components in cold temperatures is the problem of excess length between the fibers and polymer components caused by such shrinkage. For example, in addition to the cold temperature shrinkage of upjacketing on the CSM (Central Strength Member), the other polymers of the cable, such as the jacket and buffer tubes, also experience significant cold temperature shrinkage. In the prior art, the polymers used for such components, (jacket and the buffer tubes) may include PE (polyethylene), PVC/FRPVC (polyvinylchloride/flame retardant polyvinylchloride), FEP (Fluorinated Ethylene Polymer), PVDF (Polyvinylidene Fluoride) etc. . . . , each of which can shrink/contract a good amount, eg. between 0.2% to 1.5% shrinkage/contraction through the transition from room temp (23° C.) to cold temperatures (to −40° C.). The UV coated glass fibers, loosely contained in the buffer tubes, also contract in the cold, but to a much lesser extent, e.g. approximately 0.08%-1%. Moreover, the Aramid in the outer layer actually undergoes a moderate expansion in cold temperatures. The GRP of the CSM, a composite of glass and plastic, only contracts a slight amount, e.g. approximately 0.04%. The Glass Reinforced Polymers (GRPs), being bound with the plastic upjacket and tubes, slows and restricts the plastic contraction (of the upjacket and tubes), and conversely the plastic of the tubes and upjacket amplifies the GRP's contraction by a balance of forces.
The contraction in cold temperatures of these various polymer items ultimately results in the fibers having excess length relative to the tubes they are contained within. As noted above, the fibers may assume an exacerbated sinusoidal shape and even possible sharp angled bends when the available interior space within the buffer tubes is consumed by the contraction. Thus, as the temperature continues to go down, the attenuation in the fibers goes up resulting in the cable eventually failing or exceeding its desired threshold. In certain prior art designs, this situation can be further exacerbated when aramid yarns (strength elements) are used since that further limits the available free space in the buffer tubes leaving less room for adjustment.